The present invention relates to thermocouples, and in particular, to thermocouple connections. Thermocouples are a widely used type of temperature sensor. At a basic level, a thermocouple comprises positive and negative leads, made of dissimilar metals, coupled together at one end, commonly called a process end, process junction, hot end, hot junction, or main junction. The other end is commonly called a cold end, or a cold junction. Typically, the hot junction is exposed to an environment with an unknown temperature and the cold junction is exposed to an environment with a known temperature. Electrical voltage is then measured across the cold junction.
Measuring voltage across the cold junction in this manner indicates a temperature difference between the unknown temperature and the known temperature. This is because any metal will generate a voltage when exposed to a thermal gradient. A given metal tends to generate a predictable voltage as a function of the thermal gradient magnitude; however, different metals generate different voltages when exposed to the same gradient. Therefore, a thermocouple made from two different metals will create two different voltages between the hot junction and the cold junction. Thus, a measurable voltage drop from one cold junction end to the other cold junction end will reliably correspond to a temperature difference between the unknown temperature and the known temperature.
Voltage measured by the thermocouple necessarily depends on which metals are chosen for the positive and negative leads. For example, a type K thermocouple uses chromel for the positive electrode and alumel for the negative electrode. Other metals may be used instead; however, properties of each metal must be known in order to know how to convert the measured voltage into temperature.
In traditional applications, the cold junction of the thermocouple was maintained at a constant known temperature such as 0 degrees C. When the cold junction is in an environment where temperature can vary, temperature at the cold junction can be measured with another thermally sensitive device such as a thermistor, diode, or a resistance temperature detector. An appropriate correction can then be applied to each measurement by the thermocouple to adjust for the varying temperature at the cold junction. This technique is commonly referred to as cold junction compensation.